1. Field of the Invention
The present invention relates to an automatic contrast limiting circuit (ACL) and a method thereof, and more particularly to an automatic contrast limiting circuit and method thereof with a spatial domain infinite impulse response filter.
2. Description of the Related Art
Among image related consumer electronics, such as televisions, liquid crystal displays, digital cameras or personal computer cameras in hardware wise; and Photoshop and various image processing software in software wise, all of the above have the contrast adjusting function for improving image quality. The contrast adjusting formula is usually stated as below: Yo=Yi*contrast . . . (1) Wherein Yi represents the input signal and Yo represents output signal. The contrast applied to different input and corresponding output signals is illustrated as curves in FIG. 1.
In early years, contrast is adjusted according to user's preference. Generally, when the image is dark, the contrast value may be set to high so that when the brightness of the input signals is increased, picture saturation occurs as the curves A and B demonstrated in FIG. 1. On the contrary, when the image is bright, the contrast value is reduced so that poor picture definition occurs as the curves E and F demonstrated in FIG. 1.
Ideally, picture contrast can be dynamically adjusted according to input signals. Automatic Contrast Level, i.e. ACL is, therefore devised. The principle of contrast adjustment is to maximize contrast value as possible before saturation occurs. Therefore, a predetermined threshold (e.g. “t” shown in FIG. 1) and an input-related reference value is are provided to determine contrast maximum. For the time being, generally an average picture level (APL) serves as the reference value. That is, when the input signal of the image brings out bright, APL is high and the contrast value is set to be low for avoiding the picture saturated. Whereas when the input signal of the image brings out dark, the contrast value is set to high for improving picture definition.
However, the APL method is not applicable to circumstances when background is dim, such as being indoor, in the night, in a basement, in a tunnel, in a cave or in a forest. Since picture background is low in brightness, flashlight is generally applied so as to brighten up photographing object. Yet providing the object is found relatively small to the picture area, the APL value remains low, which results in a relatively higher contrast setup referring to this APL. Therefore, the object is over saturated and the image is thus poorly defined.